Conventionally, there is a technology to detect a state in which a person sits on a vehicle seat and to control a variety of safety devices such as a seatbelt or an airbag installed in a vehicle in order to improve performances of these safety devices. A film-like membrane switch having flexibility can be used as a weight sensor suitable for detecting such a seating state of a person. A seat portion and a backrest portion of the vehicle seat are generally obtained by covering a pad made of urethane or the like with a skin member such as fabric, synthetic leather or leather. More specifically, the skin member is inserted into a concave groove provided on a surface of the pad, and the inserted skin member is fixed to an anchor member (e.g. wire or resin rod) disposed adjacent to the concave groove by using a fixing tool (fixing ring). Accordingly, the skin member fits to the geometry of the surface of the pad.
Since the concave groove is located most often on a seated part of the pad (i.e. the seat portion) in the vehicle seat, a conduction portion of the weight sensor (membrane switch), which is located between detection portions of the weight sensor and conveys signals of the detection portions, extends across the concave groove. When the conduction portion is fixed and attached to the vehicle seat across the concave groove, a load on the vehicle seat may cause deformation and breaking of the conduction portion with deformation of the concave groove. Thus, the conduction portion is loosely provided in a part of the concave groove in order to prevent the breaking, and a loose part of the conduction portion is arranged inside the concave groove. However, when a person sits on the vehicle seat, the concave groove is compressed in its depth direction, in other words, the depth of the concave groove becomes low. Accordingly, the loosed part of the conduction portion may be bent extremely or be in contact strongly with another loose part. Repetition of such state may cause breaking of the conduction portion of the weight sensor.
For example, Patent Document 1 (JP 2002-211297 A corresponding to US 2002/0093236 A1) discloses a technology to prevent such breaking of a conduction portion. The technology of Patent Document 1 relates to an attachment structure of a seat sensor that is inserted between a foam and a skin of a seat on that a person sits, and detects a load on the seat. A signal line of the seat sensor extends across a concave groove provided on the foam. A wire attached to the skin extends through the concave groove, and the signal line of the seat sensor extends under the wire. A part of the concave groove overlapping with the signal line is formed to have a relatively deep bottom surface, thereby providing a deep groove part. The wire provided in the foam along the concave groove is removed from the deep groove part, and a folded portion of the signal line is arranged in the deep groove part in such a way as to have flexibility.
In Patent Document 1, the signal line is elongated to the deep groove part having a relatively deep bottom and bypasses a suspended bag. Therefore, the signal line may become long excessively, and a cost of the sensor may thereby increase. Additionally, attachment work of the sensor to the seat may become difficult.